An optical communications link can be embodied as a single end-to-end optical circuit. An optical fiber is one example of an optical link. In most cases, optical fibers are flexible, transparent fibers made by drawing glass or plastic to a suitably small diameter. Optical fibers can be used as a means to transmit light in fiber-optic communications and to permit data transmission over relatively longer distances and at relatively higher bandwidths than wire cables. Optical fibers are also preferable in some cases because signals travel along them with less loss and without electromagnetic interference.
An optical time-domain reflectometer (OTDR) is an optoelectronic instrument that can be used to evaluate an optical link or fiber. An OTDR is similar in function to a time domain reflectometer for copper wires, but used to characterize optical links. An OTDR transmits one or more optical pulses into an optical link and analyzes any light that is reflected or scattered back. The scattered or reflected light can be used to evaluate the optical link. This is similar to the way that a time domain reflectometer measures reflections caused by changes in the impedance of wires under test.